Myotonic Dystrophy Protein Kinase (DMPK) is encoded by the Myotonic Dystrophy 1 (DM1) locus. Studies on human tissues and cultured cells suggest that haploinsufficiency affecting DMPK expression is a significant consequence affecting DMPK expression is a significant consequence of the dCTG repeat expansion mutations that cause DM1. These mutations may affect other genes as well by several mechanisms. Studies of the DMPK knockout and transgenic mice have reported skeletal and cardiac myopathy, cardiac arrhythmias, and in our preliminary results, altered synaptic plasticity in the central nervous system. Knockout of the neighboring gene, Six5 or DMAHP (DM associated homeodomain protein) leads to cataracts. The CUG expansion in mRNA may sequester proteins required for splicing of the RNAs of multiple genes. On the other hand, loss-of-function mutations in other serine-threonine protein kinases can also lead to autosomal dominant traits with variable expressivity and pleiotropy such as in Coffin-Lowry and Peutz-Jeghers Syndromes. These results support continued study of DMPK with respect to understanding the pathogenesis of DM1. DMPK is also representative of a new group of serine-threonine protein kinases active in cell-cycle control, cell differentiation, and cytoskeleton organization. At least two other human protein kinases are also members. Preliminary studies in our laboratory suggest that DMPK may serve as a nexus for cross-talk or convergence between distinct signaling networks. DMPK can interact with two well known proteins that mark distinct signaling pathways: chemically stimulated Raf-1 kinase and the cytoskeleton-linked Rho family GTPase Rac-1. In order to more rigorously establish the biochemical, cellular, and physiological significance of these interactions and further understand the responsible mechanisms, we propose the following Specific Aims: 1) to study the interactions of Rac-1 and Raf-1 kinase with DMPK as purified recombinant proteins as tests for direct functional activation and synergy and their structural basis in the DMPK molecule; 2) to test for cross-talk or convergence of Rac-1 and Raf-1 kinase signaling upon DMPK in cultured myogenic and neurogenic cells in culture and verify its structural basis in the DMPK molecule; and 3) to verify cross-talk in vivo between Rac-1 and Raf-1 kinase upon DMPK in brain and muscle by constructing and studying mutant mouse lines derived from crosses of wild-type controls or DMPK knockouts with dominant negative mutant transgenes expected to alter the activation of DMPK by Rac-1 and/or Raf-1 kinase. Accomplishment of these objectives would suggest that DMPK integrates different signals in its modulation of function in muscle and nerve. These results would clearly provide a foundation for better understanding of the role that DMPK may play in the pathogenesis of DM1 and for a potentially rational design of therapeutics.